There is presently a continuous evolution of computers and above all of microprocessors. The need to process an always greater quantity of information in the shortest possible time has brought to manufacture high performance microprocessors that require high currents (which have a value of tens of amperes) for operation thereof. For this reason devices able to provide a low supply voltage and a high current have been manufactured.
However, the present microprocessors do not require high currents in continuous way but only in prefixed time periods; this is due to the quantity of operations that must be performed in said time periods. Therefore the value of the current adsorbed from them must change from tens of milliamperes to 80–100 amperes in a short time (a few nanoseconds).
Said devices are power supplies comprising switching DC-DC converters as, for example, the step-down converter shown in FIG. 1. Said converter comprises a MOS power transistor M having a non-drivable terminal connected with an inductor L that in turn is connected with a real capacitor constituted by a capacitor Cr and a resistor R and which is placed in parallel with a load LOAD, for example a microprocessor. When the load LOAD has an abrupt variation, the average current that flows through the inductor cannot rise quickly to the value required by the load LOAD and the capacitor is discharged. The waveform of the output voltage Vout, which is shown in FIG. 2a, will have a negative and positive voltage variation, respectively, at the start and at the end of the time period wherein a high current value is required by the load LOAD. If the variation of the current Iload (FIG. 2b) on the load is given by ΔI, the instantaneous voltage variation will be, in first approximation, ΔV=ΔI*R.
The switching DC-DC converters are provided with control devices normally placed between the output and the driving terminal of the power transistor of said converters. The most utilized control devices are of the analog type but control devices of the digital type are presently being affirmed which present numerous advantages with respect to the analog control devices. In fact they are less sensitive to environmental variations, are less sensitive to noise, have less sensitivity to parameter variations and also the change of the control device does not require the change of its components. The last feature allows them to have a higher flexibility with respect to control devices of the analog type because it is not necessary to change the electric components of the control device for conforming to different circuit applications.
The digital control devices are provided with an analog/digital converter able to measure the output voltage and/or current of the DC-DC converter. The information deriving from the analog/digital converter is then processed by means of a digital control algorithm. The signal generated by the algorithm is sent to the input of a PWM device the output signal of which is used to drive the power transistor of the DC-DC converter.
However, the digital control devices have a main disadvantage due to the time delay in the control loop needed for processing the information. For this reason the digital control devices that are now present in commerce do not assure that the supply voltage of the load placed downstream of the DC-DC converter is kept constant without going down a voltage level that is equal to the minimum operation value of the load when the same load requires high currents.
A digital control device for a DC-DC converter is disclosed in the article “Modeling and Simulation of new digital control for power conversion systems” Capponi, G.; Livreri, P.; Minieri, M.; Marino, F. Power Electronics Specialists Conference, 2002, pesc 02.2002 EEEE 33rd Annual, Volume 1, 2002 Pages: 155–158. In such article a technology of voltage positioning (VP) is used to minimize the excursion of the output voltage with the load variations. FIG. 3 shows the waveform of the output voltage Vo and of the current Iload in the load LOAD of the generic converter in FIG. 1 by using the control device that is present in said article: the voltage Vo shows peaks of the order of 400 mV with load current variations of the order of 15 A.